Hydrocarbon production plants utilize, after wells have been drilled through producing reservoirs, pumping systems for bringing the hydrocarbon up to the surface. For this operation, a tubular string used as a pipe for producing the hydrocarbons, at the end of which a pump is arranged, is lowered into the well.
The pumps used may have various types: piston pumps, check valve pumps, centrifugal pumps, rotary positive-displacement pumps. These pumps may be driven from the surface through rods displaced in a reciprocating or rotating motion, over the total distance between the pump and the surface, or through underwater means such as a turbine or an electric motor, the primary energy coming from the surface.
Although such pumps work satisfactorily when the fluid pumped mainly consists of liquid, problems arise and grow when a volume of free gas is present in the production fluid. In fact, if the inlet pressure of a fluid fed to a pump is lower than the bubble-point pressure above which all of the gas dissolves in oil, free gas mixed with the production fluid is allowed to pass into the pumps.
Under such conditions, the pumping efficiency is low. This problem is solved by application EP-0,435,716 in which the pump is run into an inlet pipe provided with ports at its upper end and with a check valve system at its lower end. The gas separates from the oil at the level of the ports. The check valve system protects the pump from dry running risks if the level of the hydrocarbon liquids does not reach the ports.
But in case of a viscous production fluid, the pumping device described in document EP-0,435,716 has the following drawbacks.
The viscosity of the production fluid could lead to frequent cloggings of the check valve system which make the systems ineffective. Moreover, in most geologic formations producing viscous fluids, the process of entrainment of solid particles is common, notably because of the fluid consistency. The solid particles, for example sands and sediment, near to the lower end of the inlet pipe cause clogging of the check valves.
The viscosity of the production fluid is such that the pressure drops generated inside the inlet pipe, between the inlet ports and the pump inlet, can make pumping ineffective, notably because of the decrease in the pumping rate resulting therefrom.
Similarly, the pressure drops downstream from the pump increase with the viscosity and lead to a higher power consumption upon pumping.